Multiple time programmable (MTP) memories have multiple program and erase cycling capability in comparison to the one-time program (OTP) memory. MTP memories also have the advantage of data stored therein won't disappear after the power is turned off. MTP memories have been widely used in various applications and fields such as in personal computers, electronic devices, mobile storages, and the like.
A PMOS type MTP memory requires that the dedicated voltage level of word lines to reach the target voltage at a certain rate, e.g., 1V/1 us. A too slow or too fast rate of the program voltage rise may affect the program operations of the MTP memory. Current word line voltage generator circuits utilize a long-channel PMOS transistor as a current limiter to control the voltage boosting speed. A high-voltage switch is required for each current limiter to control the boosting speed. For example, five long-channel PMOS transistors are used as a current limiter, and the number of PMOS transistors is selected by switches to implement different boosting speeds. This approach requires a fixed size of the channel length, and a switch is provided for each current limiter, so that the control of the boosting speed becomes complicated. This approach cannot obtain a uniform boosting speed, and different high-voltage power supply will also affect the boost speed control. This approach may be fast in the initial stage, but becomes slow in the final stage. This approach cannot maintain a stable 1V/1 us and cannot achieve good program operations.
Thus, there is a need for an improved word line voltage generator circuit to solve the above-described problems.